FL CDL Air Brakes cheat sheet

Compressor (engine-driven) supplies air; governor cuts it out near 120-125 psi and cuts it back in near 100 psi on most systems.

Air dryer removes moisture and oil vapor downstream of the compressor; a saturated or failed dryer sends water into the tanks and valves.

Reservoirs: wet tank receives compressor air first; dry tanks feed the service brakes; a separate supply tank feeds trailer and emergency functions.

Foot valve (brake valve) meters air pressure to the brake chambers in proportion to treadle travel; it is not a simple on/off switch.

Brake chamber converts air pressure to mechanical force; a type 30 chamber has 30 in² of effective piston area, so 100 psi yields about 3,000 lb of force.

Slack adjuster transfers chamber pushrod stroke to the S-cam; the pushrod-to-slack-adjuster angle should be about 90 degrees at full application.

Foundation brakes: S-cam drum brakes and air disc brakes; disc brakes use a caliper and are adjusted automatically.

Quick release valve at each axle exhausts air fast on release; a failed one causes slow release and dragging brakes.

Relay valve (R-12 style) is controlled by the foot valve and supplies a large volume of air to the rear axle chambers.

Dual system splits into a primary circuit (rear axle) and a secondary circuit (front axle); each has its own reservoir and check valve.

Check valves prevent air from flowing back from a charged tank to a depleted one, so one circuit can fail without draining the other.

A loss of pressure in one circuit still leaves the other circuit able to stop the vehicle; the low-pressure warning and/or buzzer must activate before pressure falls below 60 psi.

The spring (parking) brake applies mechanically when air is released from the chamber; it releases only when pressure builds above roughly 45-60 psi.

The modulator valve on vehicles with spring brakes prevents the spring brakes from applying during a service brake application, and applies them if service pressure is lost.

The tractor protection valve closes the trailer supply line at about 45 psi and the trailer service line at about 20 psi, protecting the tractor if the trailer breaks away.

The trailer emergency (supply) line charges the trailer tanks; the trailer service line carries the control signal from the tractor foot valve.

A front-brake limiting valve reduces front brake pressure in the bobtail (unloaded tractor) condition to reduce lockup risk.

An alcohol evaporator or air dryer is the moisture-control device; a failed air dryer leaves water that can freeze and block valves in cold weather.

With the engine off and the system fully charged, the static leak test: release the parking brake, hold the foot valve fully applied for one minute, and the pressure drop must be no more than 3 psi.

With the engine off and the parking brake released, hold the foot valve applied and check for a pressure drop of no more than 3 psi in one minute; also check the tractor and trailer lines for leaks.

With the engine running at fast idle, the system should build from 85 to 100 psi within 45 seconds on a typical tractor-trailer; slower build means a compressor or leak problem.

With the engine off and the parking brake released, the low-pressure warning must come on before pressure drops below 60 psi.

With the engine off and the parking brake released, the spring brakes must fully apply by the time pressure falls to about 45 psi; the vehicle must not roll.

With the parking brake released and the engine running, pump the foot valve and the spring brakes must not apply; this checks the modulator valve.

With the engine off and the parking brake released, the tractor protection valve must close the trailer supply line at about 45 psi and the trailer service line at about 20 psi.

Check the slack adjuster pushrod travel against the manufacturer limit; a typical limit is 1 inch for a type 30 chamber at 90 psi, and a pushrod stroke over the limit is an out-of-service defect.

Check the air compressor drive belt for cracks, glazing, and correct tension; a slipping belt causes slow build and low pressure.

Watch the dash gauge: normal operating pressure is roughly 100-125 psi; pressure that keeps falling while driving signals a leak.

The low-pressure warning light and buzzer must activate before pressure falls below 60 psi; a warning that never sounds is a defect.

The pressure gauge on a dual system has two needles (or two gauges), one per circuit; compare them to spot a failing circuit.

The air dryer purge cycle should occur roughly every 30 seconds at highway speed; frequent purging indicates a leak or a worn compressor.

Drain the wet tank and the dry tanks daily; water or oil in the drain indicates a failed dryer or a worn compressor.

The parking brake knob pops out automatically when system pressure drops to about 45 psi; a knob that stays in is a defect.

A low-pressure warning that activates and then stays on after pressure builds indicates a faulty sender or a real leak; do not ignore it.

The safety valve on the wet tank relieves pressure at about 150 psi; a governor that fails to cut out can cause the safety valve to vent.

If the low-pressure warning sounds while driving, stop as soon as it is safe, set the parking brake, and do not continue until the leak is found.

If a circuit fails, the remaining circuit still stops the vehicle; expect longer stopping distance and a hard pedal, and reduce speed.

If the spring brakes apply while moving, do not release the parking brake knob; instead, keep the engine running to build pressure, or use the manual release on the chamber to move the vehicle off the road.

If the compressor fails, the reservoirs hold enough air for a limited number of brake applications; use them sparingly and stop early.

If the trailer breaks away, the tractor protection valve closes the lines and the trailer spring brakes apply automatically.

If a brake chamber diaphragm ruptures, that wheel loses braking; a continuous air leak and a dragging or dead wheel are the signs.

If the air dryer freezes in cold weather, moisture reaches the valves; use the alcohol evaporator or a heated dryer, and drain tanks before the trip.

Never use the parking brake as a service brake at highway speed; a spring brake application at speed can cause a skid and loss of control.

If the brake pedal goes to the floor with no air, the spring brakes apply automatically; steer to the shoulder and stop.

Total stopping distance = perception distance + reaction distance + brake lag distance + effective braking distance.

Perception and reaction time together are about 1.5 seconds; at 55 mph that covers about 121 feet before the brakes are even applied.

Brake lag on an air brake system is about 0.4-0.5 seconds; at 55 mph that is roughly 32-40 feet of travel before the brakes take hold.

Effective braking distance grows with the square of speed: doubling speed roughly quadruples the braking distance.

Threshold braking: apply firm, steady pressure just short of lockup; do not pump the pedal on a vehicle with ABS.

With ABS, hold firm, continuous pressure and steer; ABS cycles the brakes and the pedal may pulse.

Skids: release the brake to regain rolling traction, steer in the direction of the skid, and look where you want to go.

On a downgrade, select a gear low enough that the engine and brakes hold the speed; use the brakes in short, firm applications to avoid overheating.

Brake fade from overheated drums or discs reduces stopping power; stop and let the brakes cool rather than increasing pedal pressure.

Out of service: any brake chamber pushrod stroke beyond the adjustment limit; typical limits are 1 inch for a type 30 chamber at 90 psi.

Out of service: any missing, broken, or disconnected brake component, or a brake that does not apply at all.

Out of service: air pressure that builds too slowly (below 85-100 psi in 45 seconds at fast idle) or a compressor that fails to build pressure.

Out of service: a leak that causes the pressure to drop more than 3 psi in one minute with the foot valve applied and the engine off.

Out of service: a low-pressure warning that does not activate before pressure falls below 60 psi, or spring brakes that do not apply by about 45 psi.

Out of service: a parking brake that does not hold the vehicle, or a parking brake knob that does not pop out at low pressure.

Out of service: cracked, broken, or missing brake drums or rotors; a drum with a crack that reaches the edge is a defect.

Out of service: air lines that are chafed, cracked, or leaking, and a defective air dryer that lets water into the system.

Out of service: a brake hose or line that touches a moving part, or a slack adjuster that is broken or out of adjustment.